The overall research goal is to test the effectiveness of various CO₂ thickener compounds that can induce very large changes in CO₂ viscosity at typical injection and reservoir conditions associated with Carbon Dioxide Enhanced Oil Recovery (CO₂-EOR).

The study consists of two phases. Phase 1 objectives are (1) to obtain commitment letters from CO₂-EOR operators to provide oil, brine, and core field samples and information on field operating conditions for active or planned CO₂-EOR floods, and (2) to develop laboratory testing plans for Phase 2.  The Phase 2 objective is to assess the performance of compounds that demonstrate the ability to both dissolve into and thicken CO₂ in order to reduce CO₂ mobility and increase oil recovery over a wide range of operational and field conditions in core tests

University of Pittsburgh, Pittsburgh, PA 15260

Although large-scale CO₂-EOR is practiced domestically, the potential for expansion is enormous. The single greatest obstacle to fully realizing that potential is the inherently poor volumetric sweep efficiency of the process. The very low viscosity of high-pressure CO₂ is problematic for EOR projects because it exacerbates CO₂ gravity override and induces viscous fingering, early breakthrough, poor sweep efficiency, and high CO₂ injected to oil recovered ratios, especially in formations containing relatively uniform layers of rock.

Most of the CO₂-EOR projects in the U.S. are in carbonate reservoirs, which tend to have high-permeability layers or networks of very high permeability fractures intermixed with low permeability layers or zones. Low viscosity CO₂ causes conformance control issues; a significant portion of the injected CO₂ flows into the higher permeability, watered‐out zones while a much smaller fraction of the CO₂ enters the lower permeability, oil-bearing zones of interest. The result is very low sweep efficiencies in low permeability zones during EOR operations.

The U.S. Department of Energy (DOE) sponsored an extensive literature review of strategies for improved mobility and conformance control during CO₂ floods. The findings indicated that the state‐of‐the‐art technique for mitigating unfavorable mobility ratios remains the water-alternating‐gas (WAG) process. Rather than implementing WAG or surfactant-alternating-gas (SAG) processes that require substantial amounts of water in an attempt to lower gas permeability, the University of Pittsburgh intends to dissolve a dilute (<1wt.%) amount of a “thickener” or “viscosifier” into the CO₂, thereby yielding a transparent, thermodynamically stable, high-pressure CO₂‐rich phase that is significantly more viscous than pure CO₂. This research is being funded through a DOE ARPA-E project which, if successful, will provide the compounds for testing under NETL’s FE0010799 project.

The focus of the project is to design, synthesize, and characterize a CO₂ thickener that costs less than $10/lb. and can be manufactured on a large scale. By dissolving a dilute amount of a “thickener” or “viscosifier” into the CO₂, a transparent, thermodynamically stable, high-pressure CO₂‐rich phase is created that significantly increases viscosity over pure CO₂. An increase in CO₂ viscosity should reduce problems with CO₂ gravity override, viscous fingering, production well early CO₂ breakthrough, poor sweep efficiency, and high injected CO₂ to oil recovered ratios.

More than 90 percent of CO₂-EOR floods employ water-intensive WAG processes for mobility control, creating a wide market for a CO₂ thickener. A CO₂ thickener has long been recognized as a game‐changing, transformative technology because of its potential to eliminate water injection for mobility control. Some of the remaining 10 percent of CO₂-EOR projects that do not employ WAG are still plagued by mobility control issues. Therefore, the design of an economic CO2 thickener remains an extremely relevant research aspiration. These factors contribute to increased oil recovery, better recovery economics, and fewer environmental impacts.

During Phase 1,the initial phase of this project,various CO₂ EOR operators were contacted by email, phone, and during in-person meetings. Written commitments for field samples (cleaned cores, oil, brine) and details of reservoir conditions were received from four companies. The University of Pittsburgh (Pitt) had established relationships (especially with Kinder Morgan and Denbury Resources) that would have facilitated field trials, if the thickener was successfully developed.

A one-year no-cost extension on Phase 1 of this project allowed the researchers to continue developing the best thickener to the fullest extent under ARPA-e funding, prior to conducting the Phase 2 core testing associated with this NETL award. The design, synthesis, and purification of CO₂ thickeners and initial assessments of their CO₂ solubility and ability to thicken CO₂ were conducted until April 30, 2016, under separate ARPA-e funding.

Phase 2 began on January 1, 2016, and extended to September 30, 2017. The ARPA-e-sponsored thickeners were designed to be small associating molecules that aggregate in solution to induce large increases in viscosity at low concentration. Pitt generated several effective small molecule CO₂ thickeners with ARPA-e funding. Unfortunately, none of these small molecule thickeners could dissolve in CO₂ without the addition of unacceptably large amounts of hexane or toluene as a co-solvent (e.g. 20wt% hexane, 80wt% CO₂). Therefore, no small molecule thickeners were viable candidates for Phase 2 of this award.

During Phase 2, extensive core testing of the most promising polymeric CO₂ thickener was completed. Pitt researchers verified CO₂ solubility with a phase behavior cell and the thickening potential of all polymer samples with a falling ball viscometer and a falling cylinder viscometer. The researchers planned the core tests that were conducted at Special Core Analysis Laboratories, Inc., (SCAL) in Midland, TX., and interpreted the core flooding results. The researchers also generated several fluoroacrylate homopolymers (PFA) and polyFAST polymers via bulk polymerization (mixing monomer and initiator and heating, then separating the polymer from small amounts of un-reacted monomer).

The PFA polymer was shown to impact reasonable improvements in mobility control during the SCAL core tests; for example two different viscometers were used to show that the CO₂ viscosity increased by a factor of about 3.5 when 1wt% PFA was dissolved in the CO₂. However, there was clear and surprising evidence of dramatic reductions in core permeability due to PFA adsorption, especially for sandstones. In turn, researchers realized that the CO₂-PFA solution could greatly reduce the permeability of an isolated “thief zone.” These effects were more dramatic for sandstone than for limestone. Therefore, these fluoroacrylate polymers can serve as a CO₂-soluble conformance control agent for CO₂-EOR, especially in sandstone formations. This injection of a single phase solution of CO₂ + PFA for permeability reduction is likely the first report of a CO₂-soluble conformance control additive. The researchers also demonstrated that the optimal strategy for using CO₂+PFA solutions for conformance control is analogous to the application of water-based polymeric gels; the CO₂+PFA solution should first be injected only in an isolated thief zone to induce dramatic reductions in permeability, only in that thief zone, and then CO₂ should be injected into all of the zones.

Finally, it was noted that given the propensity of PFA to adsorb onto sandstone, the adsorption of PFA from CO₂-PFA solutions onto cement surfaces may be capable of sealing cracks in casing cement that other remediation fluids (wet cement, solids-free resin, viscous aqueous emulsions) may have trouble accessing. Therefore, at the end of the project the research team reported on two proof-of-concept experiments for sealing cracked cement samples; the cracked samples had permeability of 81 nanoDarcies and 89 microDarcies. The results indicated that these small cracks could be completely sealed as CO₂+PFA solutions flowed through them via the adsorption of PFA.

The project ended on September 30, 2017. The final report will be available under "Additional Information" when it is completed.

$747,997

$284,139

NETL – Bruce Brown (bruce.brown@netl.doe.gov or 412-386-5534)
University of Pittsburgh – Robert Enick (rme@pitt.edu or 412-277-0154)